1. Field of the Invention
The present invention relates generally to step motors and, more specifically, to a method for controlling a step motor that employs velocity feedback for damping the response of the step motor.
2. Description of the Prior Art
Step motors are electromagnetic incremental motion actuators which convert digital pulse inputs to an analog output motion. The major advantage of a step motor is that a step motor can execute one step of mechanical rotation for each digital step command without using feedback. When operated without feedback, the single step response of the step motor can be quite oscillatory and may take 100 milliseconds for the oscillations to settle down. For any stepping motor system that does not use feedback, there are several step command situations where the motor will not step correctly.
One such situation occurs at the natural frequency of the step motor. When the step rate is relatively low and close to its natural frequency or at some submultiple of that frequency, the step motor can lose steps. This is because the rotor inertia and too little friction can cause a single-step response to be excessively oscillatory. In addition, at any speed the single step response of the motor is oscillatory and this causes inaccuracies in the position as well as extends the motion time. Accordingly, there is a need for a damping technique which can minimize this undesirable oscillatory motion.
There have been several attempts in the prior art to provide damping to a step motor through the use of feedback. There are electrical techniques that include some type of velocity feedback provided by a tachometer and in some cases displacement or position feedback. There are also mechanical damping techniques that place friction on the motor. These electrical or mechanical techniques are typically used to damp the operation of the step motor over its entire velocity operating range or only during the final step.
U.S. Pat. Nos. 4,446,412 and 4,091,316 which are assigned to Computervision Corporation disclose devices for controlling step motors in which velocity feedback is provided by tachometers. The velocity feedback signals are converted to a command current in U.S. Pat. No. 4,446,412 by using an analog-to-digital converter. The digital feedback signals are then used to address a PROM and converted back to an analog signal before being applied to the step motor. In U.S. Pat. No. 4,091,316 the velocity feedback signals are summed with digitally generated signals and then applied to the step motor.
U.S. Pat. No. 4,286,202, which is assigned to IBM Corporation, discloses a three phase step motor having three reluctance transducers. The reluctance transducers provide feedback signals to achieve large damping such that the phase currents can be controlled when the motor is detented.